Sterically hindered phenols in negative ion mobility spectrometry-mass spectrometry.

Negative corona discharge atmospheric pressure chemical ionization (APCI) was used to investigate phenols with varying numbers of tert-butyl groups using ion mobility spectrometry-mass spectrometry (IMS-MS). The main characteristic ion observed for all the phenolic compounds was the deprotonated molecule [M-H](-). 2-tert-Butylphenol showed one main mobility peak in the mass-selected mobility spectrum of the [M-H](-) ion measured under nitrogen atmosphere. When air was used as a nebulizer gas an oxygen addition ion was seen in the mass spectrum and, interestingly, this new species [M-H+O](-) had a shorter drift time than the lighter [M-H](-) ion. Other phenolic compounds primarily produced two IMS peaks in the mass-selected mobility spectra measured using the [M-H](-) ion. It was also observed that two isomeric compounds, 2,4-di-tert-butylphenol and 2,6-di-tert-butylphenol, could be separated with IMS. In addition, mobilities of various characteristic ions of 2,4,6-trinitrotoluene were measured, since this compound was previously used as a mobility standard. The possibility of using phenolic compounds as mobility standards is also discussed.

Characterization of proton-bound acetate dimers in ion mobility spectrometry.

Ionized acetates were used as model compounds to describe gas-phase behavior of oxygen containing compounds with respect to their formation of dimers in ion mobility spectrometry (IMS). The ions were created using corona discharge at atmospheric pressure and separated in a drift tube before analysis of the ions by mass spectrometry. At the ambient operational temperature and pressure used in our instrument, all acetates studied formed dimers. Using a homolog series of n-alkyl-acetates, we found that the collision cross section of a dimer was smaller than that of a monomer with the same reduced mass. Our experiments also showed that the reduced mobility of acetate dimers with different functional groups increased in the order n-alkyl

Separation of isomeric amines with ion mobility spectrometry.

Eight selected isomeric amines were ionized using atmospheric pressure chemical ionization and atmospheric pressure photoionization producing a protonated molecule [M+H](+) for each amine. The mobility of these ions was measured by ion mobility spectrometry. The amine compound class was shown to have an important role in mobility separation of the amines. 2,4,6-collidine, N,N-dimethylaniline and N-methyl-o-toluidine with highest observed mobilities have a N-heterocyclic aromatic ring, or are tertiary or secondary amines, respectively, whereas the rest of the compounds with lower mobilities were primary amines. It is suggested that the protonated -NH2 group (-NH3(+)) interacts more with the drift gas, and therefore the primary amines have lower mobilities. The effect of the drift gas was tested by mixing argon or helium with the nitrogen drift gas. The presence of argon shifted the mobilities towards lower values, while with helium the mobility shifted towards higher values. However, in neither case did this result in better separation of the unresolved compounds.

Separation of different ion structures in atmospheric pressure photoionization-ion mobility spectrometry-mass spectrometry (APPI-IMS-MS).

This study demonstrates how positive ion atmospheric pressure photoionization-ion mobility spectrometry-mass spectrometry (APPI-IMS-MS) can be used to produce different ionic forms of an analyte and how these can be separated. When hexane:toluene (9:1) is used as a solvent, 2,6-di-tert-butylpyridine (2,6-DtBPyr) and 2,6-di-tert-4-methylpyridine (2,6-DtB-4-MPyr) efficiently produce radical cations [M](+*) and protonated [M + H](+) molecules, whereas, when the sample solvent is hexane, protonated molecules are mainly formed. Interestingly, radical cations drift slower in the drift tube than the protonated molecules. It was observed that an oxygen adduct ion, [M + O(2)](+*), which was clearly seen in the mass spectra for hexane:toluene (9:1) solutions, shares the same mobility with radical cations, [M](+*). Therefore, the observed mobility order is most likely explained by oxygen adduct formation, i.e., the radical cation forming a heavier adduct. For pyridine and 2-tert-butylpyridine, only protonated molecules could be efficiently formed in the conditions used. For 1- and 2-naphthol it was observed that in hexane the protonated molecule typically had a higher intensity than the radical cation, whereas in hexane:toluene (9:1) the radical cation [M](+*) typically had a higher intensity than the protonated molecule [M + H](+). Interestingly, the latter drifts slower than the radical cation [M](+*), which is the opposite of the drift pattern seen for 2,6-DtBPyr and 2,6-DtB-4-MPyr.